The present invention relates to a method of controlling the operation of a brushless DC (BLDC) electric motor and to a motor operable in accordance therewith.
BLDC motors are used in a wide variety of applications including as cooling fans. In operation, the coils in a BLDC motor are driven by a current flowing in one direction or the reverse direction, the direction of the current being controlled in response to signals from a sensor detecting the rotational position of the BLDC motor rotor. Typically the position sensor is a Hall effect device.
Unfortunately, over part of their speed range, BLDC motors are prone to generate significant audible noise generated in the armature of the motor by the switching off of the stator coils.
This noise can be reduced by ‘smooth’ or ‘soft’ switching between coils and by controlling the ramp up/down slopes of the trapezoidal driving waveforms for the coils. Soft switching may be achieved by connecting capacitors in parallel with the outputs of the drive circuitry or by designing the drive circuitry to drive with a defined trapezoidal waveform. The provision of a capacitor in parallel with the output reduces the efficiency of the motor.
As the absolute magnitude of the waveforms is defined by the supply voltage slope control is achieved by varying the slope duration. Excessive slope duration can reduce the efficiency of the overall system by reducing the effective drive current whilst minimal slope duration can increase audible vibration in the coils and other mechanical parts. Typically, the optimum slope duration for minimizing audible noise in the range of interest varies with rotation rate. In one solution, a fixed slope duration is adopted, the selected duration being optimized for an desired rotational rate. In another solution, the waveform generated by the Hall effect device is amplified and clipped to produce an approximately trapezoidal waveform. Such an arrangement can be used to produce a variation in the slope of the drive. At slow rotation rates, this can reduce the efficiency of the system since the waveform rises unnecessarily slowly. Another drawback is that this solution requires a linear amplification chain from the magnetic sensor to the output drivers (including both of them) which is hard to achieve especially for the output drivers. Moreover it is inapplicable in a two-coil, open drain output driver.
It is therefore desirable to provide systems and methods for controlling a brushless DC motor that at least partially overcomes or alleviates the above problems.